CFD-14-Performance Investigation of an Aircraft Wing at ... · PDF filePerformance Investigation of an Aircraft Wing at Various Cant Angles of Winglets using CFD ... Fig. 1 Demonstrating

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Performance Investigation of an Aircraft Wing at Various Cant Angles of Winglets using CFD Simulation

Dinesh Myilsamy Post-Graduate Student

Pukyong National University, Dayeon Campus, Namgu,

Busan, Korea (South) 608-737

[email protected]

Yokesh Thirumalai Under-graduate Student Kumaraguru College of

Technology, Coimbatore, Tamil Nadu.

641- 049 [email protected]

Premkumar P.S Assistant Professor

Kumaraguru College of Technology,

Coimbatore, Tamil Nadu, 641- 049.

[email protected]

Abbreviations: Cl – Coefficient of Lift, Cd – Coefficient of drag,

α- Angle of attack

Keywords: Angle of Attack, Induced drag, Vortices, Normal wing (0° Winglet)

Abstract:

Winglet is a lift augmenting device which is attached at the wing tip of an aircraft. Winglets are used to improve the aerodynamic efficiency of an aircraft by lowering the formation of an Induced Drag which is caused by the wingtip vortices. Numerical studies have been carried out to investigate the best aerodynamic performance of a subsonic aircraft wing at various cant angles of winglets. This analytical study includes NACA 4412 airfoil coordinates for the wing design and the winglet with the blended design. The design process is carried out in CATIA-V5®. Flow features of the entire wing including winglet were examined at different cant angles of winglets varying from 0°, 30°& 90° degrees at different angles of attack from -2° to 10°. Discretization and the CFD simulation has been carried out through AcuSolve®, and the Post-processing results are obtained using AcuProbe® and AcuFieldview®. We have observed among the cases of this study, wings with winglets produces higher Cl/Cd ratio performance than the normal aircraft wing without winglets. Up to certain degree of angle of attack and by further increasing to higher angle of attack its performance getting diminished. The investigated concept of variable angle winglets appears to be a promising alternative for improving the aerodynamic efficiency of an aircraft.

Introduction

HE main purpose of any winglet is to improve the aircrafts aerodynamic performance by reducing the induced drag formation at the wing tips. [1]- [25]. The term winglet was previously used to describe an additional lifting surface on an aircraft. Wingtip devices are usually intended to improve the efficiency

of fixed-wing aircraft [1]. There are several types of wingtip devices, and although they function in different manners, the intended effect is always to reduce the aircraft's drag by partial recovery of the tip vortex energy. Wingtip devices can also improve aircraft handling characteristics and enhance safety. Such devices increase the effective aspect ratio of a wing without materially increasing the wingspan. Note that an extension of span would lower lift-induced drag, but would increase parasitic drag and would require boosting the strength and weight of the wing.

It is well known that any sort of body exposed in a viscous flow experiences profile drag, whether it produces lift or not. The induced drag is a different type of drag. It is caused by the pressure imbalance at the tip of a finite wing between its upper (pressure side) and lower (suction side) surfaces. That imbalance is necessary in order to produce a positive lift force. However, near the tip the high pressure air from the lower side tends to move upwards, where the pressure is lower, causing the streamlines to curl (see Fig. 1). This three-dimensional motion leads to the formation of a vortex, which alters the flow field and induces a velocity component in the downward direction at the wing, called downwash [2-4]. The induced flow pattern causes the relative velocity to cant downwards at each airfoil section of the wing, thus reducing the apparent angle of attack. The lift vector is tilted backwards and a force component in the direction of the drag appears, called induced drag. Reducing the size of this tip vortex and minimizing the induced drag is of great importance for the modern aircraft designers. For this purpose designers developed the winglet concept. Winglets are specially designed extensions adjusted to the wingtip that alter the velocity and pressure field and reduce the induced drag term, thus increasing aerodynamic efficiency.

T


Fig. 1 Demonstrating the tip vortex of a fixed wing aircraft.

Bourdin et al., [5] reported that the investigated concept of variable-cant-angle winglets appears to be a promising alternative to conventional control surfaces such as ailerons, elevators, and rudders as far as basic maneuvers are concerned. The concept consists of a pair of winglets with adjustable cant angle, independently actuated and mounted at the tips of a baseline flying wing. A potential application for the adjustable winglets would be for surveillance aircraft, for which enhanced low-speed maneuverability is required. Note that deflecting a winglet when the wing is flying near its stall angle is unlikely to cause the wing to stall (in contrast to the effect of an aileron). Hence, variable cant-angle winglets can be used for effective low-speed roll control (instead of spoilers which are traditionally preferred to ailerons in that flight regime).

Fig. 2 Front view of a fixed wing aircraft with fixed winglet.

Fig. 2 shows the front view of a typical aircraft with winglet at fixed cant angle. Numerical and experimental studies conducted by the earlier investigators on a flying wing configuration showed that adjustable winglets enable control moments about multiple axes, forming a highly coupled flight control system, which is in contrast to conventional control surfaces, which form a decoupled control system. Although many studies have been carried out for winglets design a generalized geometry is still not proposed by any aircraft designer under variable flying conditions [1]-[25]. In this paper diagnostic investigation of aircraft performance at different winglet cant angles has been carried out to examine the best cant angle for the winglets at variable lucrative flying conditions. Process Methodology (details with figures) Designing Design process of the aircraft wing with and without winglets and the computational flow domain have been carried out using CATIA V5 with the following specifications as mentioned below in the table 1 & 2.


SI. No.

Description Dimension

01 Airfoil Type NACA 4412

02 Wing Type Swept Back

03 Sweep Angle 19.03°

04 Wing Span 22 cm

05 Taper Ratio 0.27

06 Aspect Ratio 3.7

07 Wing Area 130.8 cm2

08 Maximum Chord 9.4 cm

09 Minimum Chord 2.5cm

Table 1 Specifications of the Aircraft Wing

SI. No

Description Dimension

01 Winglet Type Blended Winglet

02 Winglet Span 2 cm

03 Winglet Height (30° & 90°) 1 cm & 2 cm

04 Winglet Area 5 cm2

05 Winglet Sweep Angle from the wing tip

47.73°

06 Winglet Taper Ratio 0.12

07 Maximum Chord 2.5 cm

08 Minimum Chord 0.3 cm

Table 2 Specifications of the Winglet


Fig. 3 Physical model of an aircraft wing and the wing with winglets of 30° & 90° cant angles. Meshing and Pre-Processing The meshing/Discretization process was carried out using AcuConsole using global mesh attributes for the cases, 30° & 90° with reference to the normal wing for the complete fluid domain using Unstructured tetra elements are selected for these computations. As per the boundary layer calculation, boundary layer thickness was calculated as 10 mm and 12 layers are used inside the boundary layer for both the cases. The mesh details of the winglet cases are mentioned in the Table 3 & 4.

Global Mesh Attributes

Mesh size type Absolute and Relative

Absolute Mesh size

Relative Mesh Size

0.01 cm

0.125

Curvature angle 10 degree

Curvature mesh size factor 0.9

Table 3. Details of global Mesh Attributes


Table 4. Mesh Details

Figure 4 Grid system of the computational flow domain

SI. No Mesh Details Values

01 Number of Grids 3368078

02. Number of Nodes 743666

03 Element type Unstructured tetra element

04. Minimum cell size 0.001 cm

05. Maximum Cell size 1 cm

06 Number of Layers 12


Fig. 5 3D-Grid generation of the model

Boundary Conditions Numerical calculations have been carried out through AcuSolve with the help of 3-D steady state pressure based K-ω turbulence model using AcuSolve. This model uses control volume based technique to convert the governing equations to algebraic equations. The wing geometric variables and material properties are known a priori. Initial wall temperature and inlet velocity are specified. At the exit outflow boundary condition is prescribed as an outflow. At the solid walls no slip boundary condition is imposed. Ideal gas was selected as the working fluid. Inlet velocity is taken as 50 m/s and the convergence factor is set as 0.00001. The following boundary conditions mentioned below in the table 4 was used for the Numerical simulation.


SI. No Boundary Condition

01 Model K-ω Turbulence Model

02 Fluid Ideal gas

03 Flow Condition Steady state

04 Inlet Velocity Inlet = 50 m/s

05 Outflow Pressure Outflow

06 Symmetry Symmetry

07 Fairfield Wall

08 Top and Bottom Wall Wall (No-Slip)

09 Convergence factor 0.00001

10 Solver Pressure Based

Table 4. Boundary condition


Results & Discussions

It is well known that winglets application is one of the most noticeable fuel economic technologies on aircraft. The performance investigation reveals that the winglet designs must be optimized to be able to get maximum benefits during cruise and non-cruise flight conditions. In this paper comprehensive numerical studies have been carried out to examine the best aerodynamic performance of subsonic aircraft at different winglet cant angles using a validated 3D k-ω model. In the parametric analytical studies NACA series of airfoils are selected. Basic design of the winglet is selected from the literature and flow features of the entire wing including the tip effects have been examined with different cant angles of 30 and 90 degree with reference to the normal wing or 0°winglet at different angles of attack from -2°to 100

0 4 8Angle of Attack (α)

-0.2

0

0.2

0.4

0.6

0.8

Coe

ffic

ient

of

lift

(C

l)

Cl_ 0° Winglet

Cl_30° Winglet

Cl_90°winglet

Cd_ 0° Winglet

Cd_30°winglet

Cd_90°winglet

0

0.04

0.08

0.12

Coe

ffic

ient

of

Dra

g (C

d)

Fig. 6 Comparison of Cl and Cd at different angles of attack



-4

0

4

8

12

Coe

ffic

ient

of

lift

to d

rag

rati

o (C

l/Cd)

Cl/Cd _90° Winglet

Cl/Cd_0° Winglet

Cl/Cd_30° Winglet

Cl/Cd Vs. Angle of attack for 0°, 30°, 90° Winglets

90° Winglet Cl/Cd MAX = 11.18



Fig. 8 Comparison of Cl and Cd at different angles of attacks for the Normal wing

Fig. 7 Comparison of Cl/ Cd ratios at different angles of attack


-0.2

0

0.2

0.4

0.6

0.8

Coe

ffic

eint

of

lift

(C

l)

0

0.04

0.08

0.12

Coe

ffic

ient

of

Dra

g (C

d)

Cd

Cl

Cl ,Cd Vs. Angle of attack for the 0° Winglet



-0.2

0

0.2

0.4

0.6

0.8C

oeff

icei

nt o

f li

ft (

Cl)

0

0.04

0.08

0.12

Coe

ffic

ient

of

Dra

g (C

d)

Cd

Cl


Fig. 9 Comparison of Cl and Cd at different angles of attacks for the 30° Winglet

Fig.10 Comparison of Cl and Cd at different angles of attacks for the 90° Winglet


-0.2

0

0.2

0.4

0.6

0.8

Coe

ffic

eint

of

lift

(C

l)

0

0.04

0.08

0.12

Coe

ffic

ient

of

Dra

g (C

d)

Cd

Cl



Fig. 6 shows the comparison of lift coefficient (Cl) and drag coefficient (Cd) at different angles of attack without and with winglet orienting at two different cant angles viz., 300 and 900. It is evident from Fig. 6 that a case with cant angle 900 is giving the highest coefficient of lift at various angles of attack (-2°- +10°). Nevertheless, as evident in Fig. 7, this trend is not seen while comparing the coefficient of lift to drag ratio (Cl

/ Cd) at different angles of attack. One can discern from Fig. 7 that a case with 900 cant angle (Cl / Cd) is relatively high up to 20 than a case with 300 cant angle and further it lowers at 40 angle of attack and again it increases due to change in flow features. It is evident from Fig. 7 that aerodynamic performance of an aircraft with winglet at a cant angle of 300 is giving better performance up to an angle of attack 20 is giving better performance due to the change in overall flow features and the corresponding drag coefficient variation. Figs.11-37 show the pressure and velocity contours at two different cant angles and various angles of attack.

In the parametric analytical studies NACA series of airfoils are selected. Basic design of the winglet is

selected from the literature and flow features of the entire wing including the tip effects have been examined with different cant angles varying from 300 & 900 with reference to the normal wing at different angles of attack up to -2° to 100. We have observed, among the cases considered in this study that a case with 300 & 90° cant angle the aerodynamics performance of the subsonic aircraft during takeoff was found better up to 20 angles of attack and further its performance got diminished at higher angles of attack. From the observation of Cl / Cd ratio graph (fig.7) for 300 & 900 after the angle of attack 20 the 900 cant angle causes non linear variation of Cl / Cd ratio due to the turbulence effect but in the case of 300 cant angle it shows a linear variation of cl/cd ratio. Analyses further revealed that increasing the winglet cant angle from 300 to 900 at higher angles of attack could negate the performance deterioration and additionally it has enhanced the peak value of Cl / Cd on the order of 25%. A winglet’s main purpose is to improve performance by reducing drag. To understand how this is done, it is first necessary to understand the distinction between profile drag and induced drag. Profile drags is a consequence of the viscosity, or stickiness, of the air moving along the surface of the airfoil, as well as due to pressure drag (pressure forces acting over the front of a body not being balanced by those acting over its rear). As a wing moves through viscous air, it pulls some of the air along with it, and leaves some of this air in motion. Clearly, it takes energy to set air in motion.

The transfer of this energy from the wing to the air is profile drag. Profile drag depends on, among other

things, the amount of surface exposed to the air (the wetted area), the shape of the airfoil, and its angle of attack. Profile drag is proportional to the airspeed squared. Note that variable cant-angle winglets in disrupts significantly the symmetry of the wing relative to its longitudinal plane, resulting in, conceivably, a more efficient method of lateral/directional control than through the articulation of discrete control surfaces. Through various parametric analytical studies we have conjectured that aircraft with variable winglets, viz., low cant angles at low angles of attack and relatively high cant angles at high angles of attack, could give better performance during takeoff and landing.


Pressure distribution over the upper and lower surface of the wing

Fig.11 Pressure contour for the Normal Wing at α= -2° Fig.12 Pressure contour for the 30° Winglet at α= -2°

Fig.13 Pressure contour for 90° Winglet at α= -2°


Pressure contour for the Normal Wing at α= 2° Pressure contour for the 30° Winglet at α= 2°

Fig.14 Pressure contour for the 90° Winglet at α= 2°


Fig.15 Pressure contour for the Normal Wing at α= 4° Fig.16 Pressure contour for the 30° Winglet at α= 4°









Velocity distribution over the upper and the lower surface of the wing

Fig.24 Velocity contour for the Normal Wing at α= -2° Fig.25 Velocity contour for the 30° Winglet at α= -2°

Fig.26 Velocity contour for the 90° Winglet at α= -2°


Fig.27 Velocity contour for the Normal Wing at α= 2° Fig.28 Velocity contour for the 30° Winglet at α= 2°

Fig.29 Velocity contour for the 90° Winglet at α= 2°











Benefits Summary By using Acusolve we could precisely observe the clear variation in the contours results at different flow condition. Observation of the pressure and velocity variation graph for the upper and lower surface of the wing, which help us to validate cl and cd using Acuprobe Acusolve is one of the powerful computational software which saves considerable time and cost. Challenges Obtaining a smooth curvature mesh at the leading edge of the wing. Plotting cl and cd graph using AcuProbe to observe the vortex formation at the tip of the wing Future Plans In future this CFD research work will be tested at cant angles form -180° to 180°. At different angles of

attacks using Acusolve as a main solver to find out the optimum cant angle for the best aerodynamic

performance of an aircraft wing

Also this design is to be modeled and to be tested in a subsonic wind tunnel and with the obtained

experimental results CFD results are to be validated.

ACKNOWLEDGEMENT:

We thank our management for providing the better computing facility to solve this huge mesh count problem

with in short period of time and also we thank Altair technical supporting team.

Conclusions The present work shows that winglets are one of the important device to achieve better aerodynamic performance of the aircraft. . This project proposes alternatives in the cant angles of the winglet from the conventional wing without winglet. An improved winglet design with at a certain cant angle will significantly yield a better performance of an aircraft by lowering the induced drag which causes tip vortices. The overall improved aerodynamic efficiency will result in reduced fuel consumption. Although performance gains achieved with winglets are only a few percent, such small differences can be of significant profit to any airline industry. Through our analytical studies we have concluded that aircraft with variable winglets, viz., low cant angles at low angles of attack and relatively high cant angles at high angles of attack, could give better performance during takeoff and landing. Winglet cant angle optimization needs to be carried out case by case. By using CFD to predict the performance of the winglets, huge amount of time and money can be saved before testing the winglet in the wind tunnel. We concluded that the investigated concept of variable-cant-angle winglets appears to be a promising alternative for improving the aerodynamic efficiency of aircraft. .

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CFD-14-Performance Investigation of an Aircraft Wing at ... · PDF filePerformance Investigation of an Aircraft Wing at Various Cant Angles of Winglets using CFD ... Fig. 1 Demonstrating